Mechanical parts in machinery often experience unwanted vibrations during operation. The undesirable vibrations can often be reduced substantially by vibration damping means. One type of vibration damping means uses a damping mass which is coupled with a spring to a mechanical part for damping the vibrations thereof. The damping mass and the mechanical part form a system of two elastically coupled masses. The mechanical part in this system is generally referred to as the main mass. In this vibrational system, an external excitation, such as a vibration, applied to the main mass will be transmitted to the damping mass via the elastic coupling. A special case is present when the natural vibrational frequency of the damping mass, which depends on the spring rigidity of the elastic coupling, is equal to the frequency of the excitation on the main mass. In that case, if there is no energy dissipation due to inelastic damping in the vibrational system, only the damping mass vibrates in opposite phase with respect to the excitation, and the vibration of the main mass is completely damped. In practical applications, however, there is almost always some inelastic damping. The effects of inelastic damping are that the vibration of the main mass is not completely damped, and that the effective damping frequency is broadened into a band around the natural vibrational frequency of the damping mass.
A vibration damping device of the type described above may be used, for example, in motor vehicles for reducing unwanted resonance vibrations of a main mass, such as a gear part or the body of the vehicle. In such applications the natural vibrational frequency of the damping device is tuned to the resonance frequency of the main mass. The vibration damping device can be used on machine components having different shapes, such as a generally flat shape or a generally cylindrical shape. An example of a machine component with a cylindrical shape is a rotary shaft.
A vibration damping device 31 known in the prior art is represented in FIGS. 7 and 8. This device 31 comprises a cylindrical fastening part in the form of a sleeve 32, and a cylindrical damping mass 33. The fastening part is used to connect the damping device to a cylindrical machine component, such as a shaft. The fastening part 32 and the damping mass 33 are coaxially arranged, and are connected together with an elastic coupling means in the form of an elastomer spring 34. The elastomer spring 34 as shown in FIGS. 7 and 8 comprises elastic coupling elements which are elastomer webs 35 with rectangular web surfaces. Opposed pairs of the webs are disposed in planes passing the axis of the cylindrical damping device.
The vibration damping device 31 can be used to reduce linear vibrations, such as bending vibrations, of a rotary machine component in either the axial direction or the radial directions. With radial vibrations the webs 35 are stressed for pressure, and with axial vibrations the webs are stressed for thrust. The pressure rigidity and thrust rigidity of a web of a given elastomer material generally has a fixed ratio, and in general the pressure rigidity is greater than the thrust rigidity. Since the natural vibrational frequencies of the damping mass depends on the rigidity of the coupling elements, the ratio of the natural vibrational frequencies of the damping mass in axial and radial directions is correspondingly fixed, and in general the natural vibrational frequency in a radial direction is higher than that in the axial direction. Due to this fixed ratio of natural vibrational frequencies, it is virtually impossible to tune the damping device 31 in the prior art to achieve simultaneous vibration damping in both the axial and radial directions. Thus the vibration damping device 31 can be optimally tuned for damping either axial or radial vibrations, but not both.
The above description of the vibration damping device 31 is mainly for illustrating the problem that prior art vibration dampers are generally optimized for one vibrational direction only. Most unwanted vibrations, however, have amplitude components in various directions. Such a case can be present, for example, in the case of a vehicle chassis which demonstrates a mixture of vibrations in all three dimensions in the resonance frequency range. Although it is sometimes possible to use several vibration damping devices on a machine component to handle vibrations in different directions, such an approach has the disadvantages of extra weight and high costs.
Other known constructions are described in, for example, German Patent 29 33 586, which discloses a vibration damping device which is provided for reducing rotary vibrations in radial directions. In order to make the vibration damping device relatively rigid with respect to torsional vibrations, the damping device has reinforcing elements in the form of sheet metal bodies.
German Patent 39 37 669 discloses a rotary vibration damper, the spring rigidity of which increases with increasing rotational speed. The adjustment of the spring rigidity is brought about by a centrifugal force adjustment of the spring elements.